1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of a gas which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a such a gas concentration measuring apparatus designed to minimize error in determining the concentration of a gas.
2. Background Art
Limiting current type gas concentration sensors are known which are used for measuring NOx contained in exhaust gasses of an automotive engine. One such gas concentration sensor includes a pump cell and a sensor cell which are made of solid electrolyte bodies. The pump cell works to pump oxygen (O2) contained in gasses admitted into a gas chamber out of the sensor and to pump oxygen (O2) of outside gasses into the gas chamber selectively. The sensor cell works to measure the concentration of NOx contained in the gasses after passing through the pump cell. The pump cell and the sensor cell are designed to produce current signals indicative of the concentration of oxygen and NOx upon application of voltage thereto.
Another type of gas concentration sensor is known which includes a monitor cell in addition to the pump cell and the sensor cell. The monitor cell works to produce an electromotive force as a function of the concentration of oxygen within the gas chamber. A control system is also proposed which controls the voltage to be applied to the pump cell of such a three-cell gas concentration sensor as a function of a difference between an actual value and a target value of the electromotive force of the monitor cell.
For example, Japanese Patent No. 2885336 discloses the above type of gas concentration sensor.
The above discussed gas concentration sensors, however, have a drawback in that unit-to-unit variation and/or aging of the sensors usually results in a change in resistance or impedance of a solid electrolyte body, thereby leading to a decrease in accuracy of determining the concentration of a gas.